Spinal fusion is defined as the joining together of two or more adjacent vertebrae through a bridge of bone for the purpose of eliminating motion between those vertebrae. One specific type of spinal fusion is known to those skilled in the art as interbody fusion and consists of fusing the adjacent vertebrae through the disc space (the space previously occupied by the spinal disc interposed between the adjacent vertebral bodies). When such a procedure is performed from the anterior aspect of the spine (from the front) that procedure is known as anterior interbody fusion.
Typically, bone grafts are placed into the disc space to position the vertebrae apart so as to create more space for the nerves, to restore the angular relationship between said adjacent vertebrae to be fused, and to provide for material that can participate in and promote the fusion process. Substrates, other than bone, such as hydroxyapatite and/or artificial spinal fusion implants may also be used.
In general the ability to achieve bone fusion appears to be related to certain metabolic biochemical factors, the quality and quantity of the osteogenic material present (bone forming material, most commonly bone itself), the surface area available for the fusion to occur over, and the stability of the construct being fused (the ability to resist unwanted motion at the fusion site).
Consistent with the foregoing, it is desirable for the surgeon to place the largest fusion implant, generally bone, within the disc space as this provides for both the greatest surface area, and fusion mass. Furthermore, the greater the area of contact, the greater the stability of the bone-graft construct, such, that the graft is less likely to migrate, to itself collapse, or conversely to penetrate into the adjacent vertebrae as the forces across the fusion site are distributed over a greater area.
The disc space can best be described as having a biological rather than a geometric shape in that the adjacent vertebral endplate surfaces are complexly biconcave in portions, convex in others, while in still other areas extremely dense portions of bone project like “pillars” almost perpendicularly from the plane of the vertebral endplates, thereby forming partial, but substantial, side walls about the posterolateral (toward the back and side) portions of the disc space, the latter being particularly pronounced in the cervical spine.
As the bone graft used for the purpose of interbody fusion must have sufficient structural integrity to support the superincumbent body weight and the forces present across the portion of the body in which the graft is inserted, generally only quite solid portions of bone can be used for this purpose. Such portions of bone can only be cut, such as with a saw, rather than molded to fit the disc space. Even for the most skilled surgeon, it is not possible to shape such grafts to precisely fit the complex contours of the vertebral endplates adjacent the disc space. Therefore, the bone grafts are generally considerably smaller in width and depth than the disc space itself so as to confine such grafts to the more relatively flat area located about the mid portion of the vertebral endplates. The term “relatively flat” is a correct description of the mid portion of the endplate in that even this region of the vertebral endplate is not truly flat, such that it is relatively rare to achieve full congruency between the machined surface of the bone graft and the biologically determined shape of the vertebral endplate. This further compromises the quality of the fusion construct in that the area of contact between the vertebrae and the graft is suboptimal with a loss of both support area and correspondingly, graft and construct stability.
Further factors tending to limit the dimensions of the graft to less than that of the disc space include, for example in the cervical spine, the danger of the graft accidentally escaping the disc space laterally (to the side), damaging the vertebral arteries and causing a cerebral infarct and the danger of penetrating posteriorly (toward the back) and injuring the spinal cord causing paralysis. Furthermore, the previously described pillars of dense bone projecting from the overall plane of the vertebral endplates in the posterolateral portions of the disc space and commonly known as either uncinate processes or the joints of Luschka, tend to block the lateral and posterior placement of the graft(s) and tend to confine and limit the placement of the graft(s) to the anterior and central portions of the disc space.
To achieve fusion, it is necessary to at least vigorously scrape the outermost layer of the vertebral endplates until bleeding occurs to encourage the fusion, which invokes a healing process of the bone. Since the vertebral endplates are generally quite strong, it is desirable to preserve this structure even while scraping into it which can not reliably be achieved by the means of the prior art. In the past, anterior interbody fusion would be performed by removing at least a portion of the intervertebral disc and then utilizing hand held, free-hand instruments including, but not limited to, osteotomes, chisels, curettes, rongeurs, and burrs to scrape and shape the vertebral endplates and vertebral bone stock, which operations would be performed generally by working on one vertebra at a time, and independent of the position of the adjacent vertebra.
As a final consideration, not only are the vertebral endplates complexly shaped, but so are the interposed discs themselves. That is, the vertebrae of the spine are generally aligned in curved, rather than straight patterns when viewed from the side. When such curves are convex forward, as they are in the cervical and lumbar spine, the vertebrae are said to be in lordosis. Such lordosis may be the result of wedging of the vertebral bodies, of the discs, or a combination of both. When lordosis is the result of a generally wedge shaped disc, it has generally proven difficult to reliably restore that overall wedged shape to the disc space itself for the purpose of fusing the adjacent vertebrae with precisely the correct amount of lordosis.
While the discussion above has been in regard to anterior interbody fusion, it may be desirable to replace a damaged or diseased disc with a flexible member, or mechanical “artificial disc”, in which situation maximizing the surface area and congruency of contact, and controlling the angular and spatial relationships of the vertebrae adjacent that disc space would still be of great importance. As to be discussed, the present invention pertains to a means and method for the preparation of the space between adjacent vertebral bodies (“the disc space”) suitable for the implantation of an artificial disc or for interbody spinal fusions.
Attempts have been made in the past to create a guided milling apparatus for use in surgery of such appendicular joints as that of the knee. For example, U.S. Pat. No. 5,486,180 issued to Dietz on Jan. 23, 1996 teaches the use of a guided milling apparatus. The Dietz apparatus is not capable of working in the spine to prepare a space between adjacent vertebral bodies and differs from the present invention in the following ways:                1) The Dietz apparatus requires that the bone be exposed end on (Col. 1, lines 34-36, Col. 2, lines 46-47, FIGS. 1, 2, and 3). In the present invention, the “ends” of the vertebrae to be prepared are the vertebral endplates which can not be exposed on end except by dislocating the vertebrae which would cause the most grievous harm to the patient.        2) The Dietz apparatus is for the preparation of a single bone at a time (Col. 1, lines 34-36, Col. 1, lines 49-50; FIGS. 1, 2, 3, 5, 7).        3) The milling end of the Dietz apparatus removes the bone parallel to the template surface (Col. 4, lines 7-9, Col. 4, lines 50-53, FIGS. 5 and 7). In the spine, there is insufficient space available within the disc space interposed between adjacent vertebrae to insert, accommodate or operate the Dietz mechanism; this would be true regardless of actual size of the Dietz device for any size that would remain workable for use in the spine.        4) The Dietz apparatus in incapable of affixing the opposed bones on both sides of the joint simultaneously, or of preparing both joint surfaces with the opposed bones in fixed relationship with each other.        5) The Dietz apparatus teaches a means for cutting across two dimensions while controlling (fixing) for depth. (FIGS. 5,27).        6) Dietz teaches that the mill end is too large to pass through the template guide surface so as to confine the mill end beneath the guide means. (Col. 3, lines 8-19, Col. 4, lines 24-53; FIGS. 5 and 7). This thus requires that the enlarged burr portion enters the bone not through its end or face, but rather on the front surface of the bone, which entrance occurs through a cut out slot, is deep to the guide plate, and with the burr spinning about an axis parallel to the longitudinal axis of the bone itself. (Col. 2 Line, 35-37, FIGS. 1 and 2).        7) The Dietz apparatus is limited to the cutting by use of a burr along a nonlinear path. (Col. 2, lines 65; Col. 3, lines 4-6; Col. 4, line 2, FIGS. 4 and 6). This is not arbitrary as the path of the burr is guided by either or both of a pivot, allowing only for a series of arcs, and/or a branched and serpentine slot system also configured to produce only a series of arcs. (Col. 2, lines 41-42, line 52, line 65 through Col. 3, line 4, and FIGS. 1, 2, 4 and 26).        
There is therefore a need for a method and means for preparing the vertebral bodies and the vertebral endplates adjacent to a disc space to be fused by interbody fusion that:                1) allows for the safe preparation of the disc space to the optimal depth and width so as to allow for the correct use of the largest possible fusion implant which would be associated with the direct benefits of providing for the maximum mass of osteogenic material, the largest possible surface area for fusion to occur over, increased graft and construct stability secondary to the increased area of contact, and the greatest protection against implant collapse or penetration into the vertebral bodies from the distribution of the loads over the greatest surface area;        2) allows for the preparation of the vertebral endplates to a known and uniform surface configuration, which configuration can be matched by a corresponding surface of the fusion implant thereby providing for the greatest possible interface congruity between the vertebral endplates and fusion implant, and providing for the optimal contact surface, enhanced fusion area, enhanced graft and construct stability, and decreased load per surface area;        3) allows for the restoration of the correct vertebral alignment by preparing the vertebral endplates in fixed relationship to each other adjacent the disc space so as to three dimensionally shape the disc space-fusion implant site;        4) allows for an efficient and reliable means for scraping the central portions of the outer layer of the vertebral endplates without the danger of removing those structures entirely; and        5) allows for the extension of the fusion area into the extremely supportive and extremely dense bone of the posterior lateral regions of the disc space.        